A Multiple-Input-Multiple-Output (MIMO) system is a system having more than one input and more than one output, in other words both the transmitter Tx and receiver Rx of a MIMO system have multiple antennas operating over the same bandwidth. This allows a MIMO system to have higher throughput for the same spectrum because of its ability to transmit and receive multiple streams of data simultaneously. A MIMO system with similar count of antennas at both the transmitter and the receiver in a point-to-point (PTP) radio link is able to multiply the system throughput linearly with every additional antenna. For example, a 2×2 MIMO will double the throughput compared to a Single Input Single Output (SISO) system.
A MIMO system may be a Line Of Sight (LOS) MIMO system or a non-LOS MIMO system. In a LOS MIMO system, a signal travels over the air directly from a transmitter to a receiver in a straight line without passing any obstructions. On the other hand, in a non-LOS MIMO system a signal passes obstructions when traveling from a transmitter to a receiver. A signal that passes obstacles on its way may be reflected, diffracted, absorbed, scattered etc., which may create multiple signals that will arrive at the receiver at different times, paths and with different signal strength.
MIMO systems for PTP radio links differs from non-LOS MIMO systems in that sense that the channels between transmit and receive antennas are close to static. A well known solution is in this case to use a canceling technique similar to what is used in Cross-Polar-Interference-Cancellation (XPIC). In this case all phase tracking may be allocated to the receive side which may be required in case of microwave frequencies where hardware related aspects such as phase noise may be more critical.
As known for a person skilled in the art, an XPIC system is an adaptive coupling electronic circuit handling the problem of cross-polarization interference. An XPIC circuit is a circuit between two orthogonal co-frequency channels used to reduce cross-polar interference during adverse propagation conditions. An XPIC system filters a cross-polarization interference signal in order to successfully receive or decode a desired signal.
FIG. 1 shows a block diagram of a simplified 2×2 non-LOS MIMO system 100, where 2×2 implies two antennas Tx1, Tx2 101 at the transmitter and two antennas Rx1, Rx2 103 at the receiver. The 2×2 MIMO system 100 equals an XPIC system. The main difference between the MIMO and XPIC is that in an XPIC application the phase difference between the two received signals is not defined, which may result in destructive phases in the addition point after the two adaptive filters 1 and 2 105. Therefore, in this case there may be a limit on the cross signal levels that may not be exceeded. However, in case of LOS-MIMO this phase difference is controlled by the antenna separation and the destructive case is avoided and thereby there is no cross level limitation.
The block diagram also comprises a Canceller-Phase Locked Loop (CLR-PLL) 108 and a symbol clock regeneration 110. The phase tracking of differential phase noise may be improved by adding the separate CLR-PLL 108. The regenerated symbol clock is a clock signal which is synchronized with the signals received at the receiver antennas Rx1, Rx2 103.
In FIG. 1 in general, signals, i.e. data modulated signals data 1 and data 2, sent from the two transmitters Tx1, Tx2 101 are received in the receivers Rx1, Rx2 103. The received signals are filtered in the adaptive filter 1 and adaptive filter 2 105. A symbol clock signal, i.e. the clock of the input digital data, is also regenerated 110 at the same time as the filtering is performed. Then, all interference between the outputs from the adaptive filters 105 is cancelled 111, i.e. using a summing point and phase rotators. In the receiver Rx1 103 the signals from both transmitter antennas Tx1 and Tx2 101 are present and the signal from the second transmitter antenna Tx2 101 should be cancelled. In order to do that, it is necessary to track the differential phase noise between the receivers Rx1 and Rx2 103.
The phase tracking of differential phase noise is improved by running the signals through the CLR-PLL 108. The error feedback generator 112 comprises several functions, such as symbol decision, symbol error detection and modification of symbol errors for filter coefficient update. In other words, it comprises generation of errors from the interference cancelled signals. The dotted box 115 comprises the same components discussed above, adaptive filters 1 and 2 105, CLR-PLL 108, symbol clock regeneration 110, canceller 111 and error feedback generator 112, but they are not repeated for the sake of simplicity. The data output from the non-LOS MIMO system 100 are data 1 and data 2, i.e. the same data as input into the system 100.
A problem with the solution shown in FIG. 1 when used as a 2×2-LOS-MIMO system is the symbol clock generation. The two received signals in each of the receivers 103 usually have equal signal level. It is difficult to control the exact clock timing difference and if no countermeasures are taken it will result in large clock jitter or even lack of clock synchronization.